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diff --git a/Documentation/mips/time.README b/Documentation/mips/time.README new file mode 100644 index 000000000000..70bc0dd43d6d --- /dev/null +++ b/Documentation/mips/time.README @@ -0,0 +1,198 @@ +README for MIPS time services + +Jun Sun +jsun@mvista.com or jsun@junsun.net + + +ABOUT +----- +This file describes the new arch/mips/kernel/time.c, related files and the +services they provide. + +If you are short in patience and just want to know how to use time.c for a +new board or convert an existing board, go to the last section. + + +FILES, COMPATABILITY AND CONFIGS +--------------------------------- + +The old arch/mips/kernel/time.c is renamed to old-time.c. + +A new time.c is put there, together with include/asm-mips/time.h. + +Two configs variables are introduced, CONFIG_OLD_TIME_C and CONFIG_NEW_TIME_C. +So we allow boards using + + 1) old time.c (CONFIG_OLD_TIME_C) + 2) new time.c (CONFIG_NEW_TIME_C) + 3) neither (their own private time.c) + +However, it is expected every board will move to the new time.c in the near +future. + + +WHAT THE NEW CODE PROVIDES? +--------------------------- + +The new time code provide the following services: + + a) Implements functions required by Linux common code: + time_init + do_gettimeofday + do_settimeofday + + b) provides an abstraction of RTC and null RTC implementation as default. + extern unsigned long (*rtc_get_time)(void); + extern int (*rtc_set_time)(unsigned long); + + c) a set of gettimeoffset functions for different CPUs and different + needs. + + d) high-level and low-level timer interrupt routines where the timer + interrupt source may or may not be the CPU timer. The high-level + routine is dispatched through do_IRQ() while the low-level is + dispatched in assemably code (usually int-handler.S) + + +WHAT THE NEW CODE REQUIRES? +--------------------------- + +For the new code to work properly, each board implementation needs to supply +the following functions or values: + + a) board_time_init - a function pointer. Invoked at the beginnig of + time_init(). It is optional. + 1. (optional) set up RTC routines + 2. (optional) calibrate and set the mips_counter_frequency + + b) board_timer_setup - a function pointer. Invoked at the end of time_init() + 1. (optional) over-ride any decisions made in time_init() + 2. set up the irqaction for timer interrupt. + 3. enable the timer interrupt + + c) (optional) board-specific RTC routines. + + d) (optional) mips_counter_frequency - It must be definied if the board + is using CPU counter for timer interrupt or it is using fixed rate + gettimeoffset(). + + +PORTING GUIDE +------------- + +Step 1: decide how you like to implement the time services. + + a) does this board have a RTC? If yes, implement the two RTC funcs. + + b) does the CPU have counter/compare registers? + + If the answer is no, you need a timer to provide the timer interrupt + at 100 HZ speed. + + You cannot use the fast gettimeoffset functions, i.e., + + unsigned long fixed_rate_gettimeoffset(void); + unsigned long calibrate_div32_gettimeoffset(void); + unsigned long calibrate_div64_gettimeoffset(void); + + You can use null_gettimeoffset() will gives the same time resolution as + jiffy. Or you can implement your own gettimeoffset (probably based on + some ad hoc hardware on your machine.) + + c) The following sub steps assume your CPU has counter register. + Do you plan to use the CPU counter register as the timer interrupt + or use an exnternal timer? + + In order to use CPU counter register as the timer interrupt source, you + must know the counter speed (mips_counter_frequency). It is usually the + same as the CPU speed or an integral divisor of it. + + d) decide on whether you want to use high-level or low-level timer + interrupt routines. The low-level one is presumably faster, but should + not make too mcuh difference. + + +Step 2: the machine setup() function + + If you supply board_time_init(), set the function poointer. + + Set the function pointer board_timer_setup() (mandatory) + + +Step 3: implement rtc routines, board_time_init() and board_timer_setup() + if needed. + + board_time_init() - + a) (optional) set up RTC routines, + b) (optional) calibrate and set the mips_counter_frequency + (only needed if you intended to use fixed_rate_gettimeoffset + or use cpu counter as timer interrupt source) + + board_timer_setup() - + a) (optional) over-write any choices made above by time_init(). + b) machine specific code should setup the timer irqaction. + c) enable the timer interrupt + + + If the RTC chip is a common chip, I suggest the routines are put under + arch/mips/libs. For example, for DS1386 chip, one would create + rtc-ds1386.c under arch/mips/lib directory. Add the following line to + the arch/mips/lib/Makefile: + + obj-$(CONFIG_DDB5476) += rtc-ds1386.o + +Step 4: if you are using low-level timer interrupt, change your interrupt + dispathcing code to check for timer interrupt and jump to + ll_timer_interrupt() directly if one is detected. + +Step 5: Modify arch/mips/config.in and add CONFIG_NEW_TIME_C to your machine. + Modify the appropriate defconfig if applicable. + +Final notes: + +For some tricky cases, you may need to add your own wrapper functions +for some of the functions in time.c. + +For example, you may define your own timer interrupt routine, which does +some of its own processing and then calls timer_interrupt(). + +You can also over-ride any of the built-in functions (gettimeoffset, +RTC routines and/or timer interrupt routine). + + +PORTING NOTES FOR SMP +---------------------- + +If you have a SMP box, things are slightly more complicated. + +The time service running every jiffy is logically divided into two parts: + + 1) the one for the whole system (defined in timer_interrupt()) + 2) the one that should run for each CPU (defined in local_timer_interrupt()) + +You need to decide on your timer interrupt sources. + + case 1) - whole system has only one timer interrupt delivered to one CPU + + In this case, you set up timer interrupt as in UP systems. In addtion, + you need to set emulate_local_timer_interrupt to 1 so that other + CPUs get to call local_timer_interrupt(). + + THIS IS CURRENTLY NOT IMPLEMNETED. However, it is rather easy to write + one should such a need arise. You simply make a IPI call. + + case 2) - each CPU has a separate timer interrupt + + In this case, you need to set up IRQ such that each of them will + call local_timer_interrupt(). In addition, you need to arrange + one and only one of them to call timer_interrupt(). + + You can also do the low-level version of those interrupt routines, + following similar dispatching routes described above. + +Note about do_gettimeoffset(): + + It is very likely the CPU counter registers are not sync'ed up in a SMP box. + Therefore you cannot really use the many of the existing routines that + are based on CPU counter. You should wirte your own gettimeoffset rouinte + if you want intra-jiffy resolution. |